100% 



56 INFRA-RED TRANSMISSION SPECTRA. 



in the sulphates examined, especially for K, Ba, Ca, and Pb, which have 

 a common band at 6.5 /*,. 



The sulphates of the metals K, Rb, and Cs have been compared by 

 Tutton, 1 who has shown that both as regards crystalline form, specific 

 gravity, thermal expansion, and corresponding refractive indices the 



Rb salt lies between the K 

 and Cs salts. These mono- 

 valent elements occupy con- 

 secutive positions in the 

 even series of Mendeleef's 

 table. The next even series 

 is Ca, Sr, and Ba, the trans- 

 mission curves of which are 

 before us. The following 

 odd series contains Mg, 

 which just precedes Ca and 

 Cd, which lie between Sr 

 and Ba. In other words, 

 the elements lie in the order 

 Mg, Ca, Sr, Cd, Ba, while 

 the maximum of the ab- 

 sorption band occurs in the 

 order 4.5, 4.55, 4.6, 4.6, 

 4.63 p. Whether this is a 

 true shift, with increase in 

 molecular weights, needs 

 further examination. The 

 data presented are certainly 

 very suggestive of a real 

 shift, with increase in mole- 

 cular weight of the metal. This same shifting of the maximum, with 

 increase in molecular, will be noticed in the reflection curves of SrSO 4 

 and BaSO 4 ; in the former the maxima are at 8.2, 8.75, and 9.05 /j., while 

 in the latter the maxima are shifted to 8.34, 8.9, and 9.1 /A. 



Drude 2 has shown that in the ultra-violet the absorption band is due 

 to the sympathetic vibrations of particles which have a charge and mass 

 identical with the "ion" (or "corpuscle"), while, in the infra-red, the 

 absorption bands are due to particles which have a mass of the order 

 of magnitude of the molecule. From this standpoint one would expect 



0123 



FIG. 42. Sulphuric acid () ; Cadmium sulphate. 



1 See Miers, Mineralogy. 



2 Drude: Ann. cler Phys. (4), 14, p. 677, 1904. 



